There are known measuring and control systems, e.g. in numerical control machine tools, for detecting the position and/or the dimensions of machined pieces by a contact detecting probe, mounted in the machine. In a system of this type, shown in simplified form in FIG. 1, a checking probe 1, for example a contact detecting probe, that, in the course of a checking cycle, displaces with respect to a piece 3 being machined, touches the surfaces to be checked and responds to contact, detected by suitable detecting devices identified with reference number 2, by wirelessly transmitting, by means of a transmitter 4, pulse signals 5—that identify the state of the probe 1—to a receiver 7, usually located at a certain distance from the probe 1. The receiver 7 is in turn connected, by means of an interface device 9, to the numerical control unit 11 of the machine that, by processing other signals indicative of the spatial position of probe 1, obtains information about the position of the surfaces of the piece 3. At times the interface device 9 can be integrated at the interior of the receiver 7.
The contact detecting probe can include electric batteries for the power supply of contact detecting circuits and of the transmitter 4 that can operate, for example, by emitting signals (5) of optical or radio-frequency type. U.S. Pat. No. 5,778,550 discloses a measuring system with these characteristics and describes a checking probe with circuits for sending suitably coded, optical signals in the infrared band, and a receiver unit including one or more photodiodes, amplification circuits and shaping circuits for reconstructing a sequence of pulses corresponding to the received optical signals. In the shaping circuits, the received and amplified signal is compared with a suitable threshold, whose value can be altered for varying the sensitivity of the receiver in the course of specific operation phases of the system.
There are also known systems with receiver units 7 that include the characteristics described in the prior art portion of claim 1, as shown in simplified form in FIG. 2, where an input section includes a receiver device, for example a photodiode 13, that receives the optical signals 5 and amplification circuits with an amplifier for example of the differential type, 15, whose output, more particularly the amplitude of the amplified signal, or input signal, is compared, in the circuits of a comparison section 20, with values of a reference signal, or threshold, for generating—and sending to the interface device 9—a sequence of pulses including the information received from the remote probe 1. Typically, the optical signals 5 are transmitted by the probe 1 as groups or trains of coded pulses, for example groups of few pulses of few microseconds. The groups occur at approximately 15-20 millisecond intervals.
The threshold is generated and dynamically varied by the circuits of a generation and control section 16, on the basis of both indications arriving from a logic 17 and attributes of the received optical signal 5.
More specifically, the logic 17 communicates to generating circuits 30 of the section 16 information relating to the specific application, for example on the basis of data that the operator has set in hardware (dip-switch) memories, and/or to particular operation phases, as briefly cited above with reference to U.S. Pat. No. 5,778,550. Dynamic variations of the threshold are instead caused by automatic control circuits, more specifically detecting circuits 40, on the basis of amplitude peaks of the input signals. In practice, the threshold is quickly varied, with respect to a maximum sensitivity value defined on the basis of the signals of the logic 17, so as to reduce its distance from the peak amplitude of the input signal, and to maintain a reduced sensitivity for a short period of time, sufficient for preventing the generation of false output pulses owing to possible signal distortions in the receiver circuits when the signal is strong.
A typical case foresees, for example, quick threshold increments (or decrements, if the threshold has negative value) until reaching values close to the peak amplitude of the input signal, with time constant in the order of the microsecond, and a return to the maximum sensitivity value within a period of time in the order of the millisecond. The time interval in the course of which the sensitivity of the receiver is diminished is sufficiently long for overcoming noises that could occur caused by the distortion of a group of pulses.
Probe receivers with these characteristics are manufactured and marketed with good results by the same applicant of the present patent application since the 90's. These receivers include, among other things, circuital components acting as high-pass filter for reducing the negative effects due to the continuous and low-frequency components of the surrounding environmental illumination and for inhibiting from subsequent processings low-frequency noise components emitted, for example, by fluorescent and incandescent lamps located in the surrounding environment where the receiver operates. The winding or inductor 14 of FIG. 2 shows, in simplified form, the previous high-pass filter. Furthermore, there can be foreseen cells for the high-pass filtering at the interior of the amplifier 15.
However, there is the possibility that radiations emitted in an unforeseeable way by fluorescent lamps or by other sources of light in the environment be processed by the receiver together with the signals transmitted by the probe thereby causing malfunctions.
It has been experienced that fluorescent lamps emit improper and unforeseeable radiations, even in the infrared radiation band, and that these radiations have considerable high-frequency amplitude modulation components, i.e. in the frequency band of the useful signals, in other terms of the pulse signals 5. These radiations vary depending on the type of lamp, on the environment temperature, on the power supply voltage, on the age and the efficiency conditions of the lamp itself.
In the known embodiment shown in FIG. 2 the maximum sensitivity is reset after the elapse of a time that is relatively short with respect, for example, to the typical time interval between groups of pulses transmitted by the transmitter 4 of the probe 1. It is possible to envisage to lengthen this time for improving immunity to noise, but this could involve the risk of loosing “good” signals, if the amplitude of these signals rapidly decreases in consequence, for example, of the probe 1 rapidly displacing away from the receiver 7.